Sorry, but that off duty officer was a pretty sad example. Could he not SEE the marks left on that boy? If his dad had just hit him, his face, chest and arms would be red. Does this kid have a mother around? I would be calling the school ASAP and telling the school EXACTLY what you saw. I would also contact CPS, the cops, just about anybody else that I could think of.
If a child is being abused, then yes it is our duties as adults to step in. If you are the coach, team captain, what have you, then you need to make it perfectly clear that screaming, cursing, and what not is not acceptable.